Liquid chromatography (LC) is a well-known technique for separating the constituent elements in a given sample. In a conventional LC system, a liquid solvent (referred to as the “mobile phase”) is introduced from a reservoir and is pumped through the LC system. The mobile phase exits the pump under pressure. The mobile phase then travels via tubing to a sample injection valve. As the name suggests, the sample injection valve allows an operator to inject a sample into the LC system, where the sample will be carried along with the mobile phase.
In a conventional LC system, the sample and mobile phase pass through one or more filters and often a guard column before coming to the column. A typical column usually consists of a piece of steel tubing which has been packed with a “packing” material. The “packing” consists of the particulate material “packed” inside the column. It usually consists of silica- or polymer-based particles, which are often chemically bonded with a chemical functionality. The packing material is also known as the stationary phase. One of the fundamental principles of separation is the mobile phase continuously passing through the stationary phase. When the sample is carried through the column (along with the mobile phase), the various components (solutes) in the sample migrate through the packing within the column at different rates (i.e., there is differential migration of the solutes). In other words, the various components in a sample will move through the column at different rates. Because of the different rates of movement, the components gradually separate as they move through the column. Differential migration is affected by factors such as the composition of the mobile phase, the composition of the stationary phase (i.e., the material with which the column is “packed”), and the temperature at which the separation takes place. Thus, such factors will influence the separation of the sample's various components.
Once the sample (with its components now separated) leaves the column, it flows with the mobile phase past a detector. The detector detects the presence of specific molecules or compounds. Two general types of detectors are used in LC applications. One type measures a change in some overall physical property of the mobile phase and the sample (such as their refractive index). The other type measures only some property of the sample (such as the absorption of ultraviolet radiation). In essence, a typical detector in a LC system can measure and provide an output in terms of mass per unit of volume (such as grams per milliliter) or mass per unit of time (such as grams per second) of the sample's components. From such an output signal, a “chromatogram” can be provided; the chromatogram can then be used by an operator to determine the chemical components present in the sample.
In addition to the above components, a LC system will often include filters, check valves, a guard column, or the like in order to prevent contamination of the sample or damage to the LC system. For example, an inlet solvent filter may be used to filter out particles from the solvent (or mobile phase) before it reaches the pump. A guard column is often placed before the analytical or preparative column; i.e., the primary column. The purpose of such a guard column is to “guard” the primary column by absorbing unwanted sample components that might otherwise bind irreversibly to the analytical or preparative column.
In practice, various components in an LC system may be connected by an operator to perform a given task. For example, an operator will select an appropriate mobile phase and column, then connect a supply of the selected mobile phase and a selected column to the LC system before operation. In order to be suitable for high performance liquid chromatography (HPLC) applications, each connection must be able to withstand the typical operating pressures of the HPLC system. If the connection is too weak, it may leak. Because the types of solvents that are sometimes used as the mobile phase are often toxic and because it is often expensive to obtain and/or prepare many samples for use, any such connection failure is a serious concern.
It is fairly common for an operator to disconnect a column (or other component) from a LC system and then connect a different column (or other component) in its place after one test has finished and before the next begins. Given the importance of leak-proof connections, especially in HPLC applications, the operator must take time to be sure the connection is sufficient. Replacing a column (or other component) may occur several times in a day. Moreover, the time involved in disconnecting and then connecting a column (or other component) is unproductive because the LC system is not in use and the operator is engaged in plumbing the system instead of preparing samples or other more productive activities. Hence, the replacement of a column in a conventional LC system involves a great deal of wasted time and inefficiencies.
Given concerns about the need for leak-free connections, conventional connections have been made with stainless steel tubing and stainless steel end fittings. More recently, however, it has been realized that the use of stainless steel components in a LC system have potential drawbacks in situations involving biological samples. For example, the components in a sample may attach themselves to the wall of stainless steel tubing. This presents problems because the detector's measurements (and thus the chromatogram) of a given sample may not accurately reflect the sample if some of the sample's components or ions remain in the tubing, and do not pass the detector. Perhaps of even greater concern, however, is the fact that ions from the stainless steel tubing may detach from the tubing and flow past the detector, thus leading to potentially erroneous results. Additionally, ions can easily bind to biological compounds of interest, resulting in changes to the molecules that affect their retention time in the column. Hence, there is a need for “biocompatible” connections through the use of a material that is chemically inert with respect to such “biological” samples and the mobile phase used with such samples so that ions will not be released by the tubing and thus contaminate the sample.
In many applications using selector/injector valves to direct fluid flows, and in particular in liquid and gas chromatography, the volume of fluids is small. This is particularly true when liquid or gas chromatography is being used as an analytical method as opposed to a preparative method. Such methods often use capillary columns and are generally referred to as capillary chromatography. In capillary chromatography, both gas phase and liquid phase, it is often desired to minimize the internal volume of the selector or injector valve. One reason for this is that a valve having a large volume will contain a relatively large volume of liquid, and when a sample is injected into the valve the sample will be diluted, decreasing the resolution and sensitivity of the analytical method.
Micro-fluidic analytical processes also involve small sample sizes. As used herein, sample volumes considered to involve micro-fluidic techniques can range from as low as volumes of only several picoliters or so, up to volumes of several milliliters or so, whereas more traditional LC techniques, for example, historically often involved samples of about one microliter to about 100 milliliters in volume. Thus, the micro-fluidic techniques described herein involve volumes one or more orders of magnitude smaller in size than traditional LC techniques. Micro-fluidic techniques can also be expressed as those involving fluid flow rates of about 0.5 ml/minute or less.
Most conventional HPLC systems include pumps which can generate relatively high pressures of up to around 5,000 psi to 6,000 psi or so. In many situations, an operator can obtain successful results by operating a LC system at “low” pressures of anywhere from just a few psi or so up to 1,000 psi or so. More often than not, however, an operator will find it desirable to operate a LC system at relatively “higher” pressures of over 1,000 psi.
Another, relatively newer liquid chromatography form is Ultra High Performance Liquid Chromatography (UHPLC) in which system pressure extends upward to about 1400 bar or 20,000 psi or so, or even more. In order to achieve greater chromatographic resolution and higher sample throughput, the particle size of the stationary phase has become extremely small. A stationary phase particle as small as 1 micron is common; the resulting high column packing density leads to substantially increased system pressure at the head of the column. Both HPLC and UHPLC are examples of analytical instrumentation that utilize fluid transfer at elevated pressures. For example, in U.S. Patent Publication No. 2007/0283746 A1, published on Dec. 13, 2007 and titled “Sample Injector System for Liquid Chromatography,” an injection system is described for use with UHPLC applications, which are said to involve pressures in the range from 20,000 psi to 120,000 psi. In U.S. Pat. No. 7,311,502, issued on Dec. 25, 2007 to Gerhardt, et al., and titled “Method for Using a Hydraulic Amplifier Pump in Ultrahigh Pressure Liquid Chromatography,” the use of a hydraulic amplifier is described for use in UHPLC systems involving pressures in excess of 25,000 psi. In U.S. Patent Publication No. 2005/0269264 A1, published on Dec. 8, 2005 and titled “Chromatography System with Gradient Storage and Method for Operating the Same,” a system for performing UHPLC is disclosed, with UHPLC described as involving pressures above 5,000 psi (and up to 60,000 psi). Applicants hereby incorporate by reference as if fully set forth herein U.S. Pat. No. 7,311,502 and US Patent Publications Nos. 2007/0283746 A1 and 2005/0269264 A1.
As noted, liquid chromatography (as well as other analytical) systems, including HPLC or UHPLC systems, typically include several components. For example, such a system may include a pump; an injection valve or autosampler for injecting the analyte; a precolumn filter to remove particulate matter in the analyte solution that might clog the column; a packed bed to retain irreversibly adsorbed chemical material; the HPLC column itself; and a detector that analyzes the carrier fluid as it leaves the column. These various components may typically be connected by a miniature fluid conduit, or tubing, such as metallic or polymeric tubing, usually having an internal diameter of 0.001 to 0.040 inch.
All of these various components and lengths of tubing are typically interconnected by threaded fittings. Fittings for connecting various LC system components and lengths of tubing are disclosed in prior patents, for example, U.S. Pat. Nos. 5,525,303; 5,730,943; and 6,095,572, the disclosures of which are herein all incorporated by reference as if fully set forth herein. Often, a first internally threaded fitting seals to a first component with a ferrule or similar sealing device. The first fitting is threadedly connected through multiple turns by hand or by use of a wrench or wrenches to a second fitting having a corresponding external fitting, which is in turn sealed to a second component by a ferrule or other seal. Disconnecting these fittings for component replacement, maintenance, or reconfiguration often requires the use of a wrench or wrenches to unthread the fittings. Although a wrench or wrenches may be used, other tools such as pliers or other gripping and holding tools are sometimes used. It will be understood by those skilled in the art that, as used herein, the term “LC system” is intended in its broad sense to include all apparatus and components in a system used in connection with liquid chromatography, whether made of only a few simple components or made of numerous, sophisticated components which are computer controlled or the like. Those skilled in the art will also appreciate that an LC system is one type of an analytical instrument (AI) system. For example, gas chromatography is similar in many respects to liquid chromatography, but obviously involves a gas sample to be analyzed. Such analytical instrument systems include high performance or high pressure liquid chromatography systems, an ultra high performance or ultra high pressure liquid chromatography system, a mass spectrometry system, a microflow chromatography system, a nanoflow chromatography system, a nano-scale chromatography system, a capillary electrophoresis system, a reverse-phase gradient chromatography system, or a combination thereof. Although the following discussion focuses on liquid chromatography, those skilled in the art will appreciate that much of what is said also has application to other types of AI systems and methods.
Increasing pressure requirements in liquid chromatography have necessitated the use of high pressure fluidic components. For many applications regular stainless steel tubing can be used to withstand the high pressure. However, for some types of analyses (e.g., biological testing and metal/ion analysis), stainless steel or other metals are not desired in the fluid path as the metal could interfere with the testing. Additionally, there are some fields of use (e.g., nano-scale or nano-volume analysis), that require very small inside diameters to accommodate the extremely low volumes required by these applications. Such small inside diameters are typically not available in stainless steel or other high pressure tubing.
In high-performance liquid chromatography (HPLC), ultra high-performance liquid chromatography (UHPLC), and other high-pressure analytic chemistry applications, various system components and their fluidic connections must be able to withstand pressures of 15,000 to 20,000 psi or so. The types of fluidic connection systems between the tubes that carry fluids and the ports that receive fluids in these high-pressure applications are limited. Many fluidic connection systems rely on cone-shaped, threaded, or welded fittings to attach a tube to a receiving port. These types of connections sometimes may have drawbacks, however. For example, the size of cone-shaped fittings and threaded fittings are dependent on the type and size of any given port, which makes quickly interchanging a tube fitted with a particular cone or threaded fitting between various ports difficult. Other compression-based fittings have been employed to address this problem. Such fittings often employ a ferrule or a lock ring to help secure one end of a tube to a receiving port. However, ferrules and lock rings can become deformed after multiple uses (e.g., by connecting, disconnecting, and reconnecting to various ports). This is especially true in high-pressure applications, where a fluid-tight seal is essential, and where a ferrule or lock ring may be more likely to become deformed in creating such a seal.
For example, published U.S. Patent Application No. 2013/0043677, titled “Tube and Pipe End Cartridge Seal,” published on Feb. 21, 2013, describes a tube and pipe end cartridge seal for use at high pressures, which relies on a fitting body (including ferrule fittings) to effectuate a seal with the axial end of a tube. Moreover, a dimple is forged on the annular end of the tube face to further effectuate the seal. Likewise, U.S. Pat. No. 6,056,331, titled “Zero Dead Volume Tube to Surface Seal,” issued to Bennett et al. on May 2, 2000, describes an apparatus for connecting a tube to a surface using a body, a ferrule, and a threaded fitting. Although Bennett et al. discloses a type of tube face-sealing apparatus, the apparatus of Bennet et al. relies on a threaded fitting and a ferrule. Similarly, published U.S. Patent Application No. 2012/0061955, titled “Plug Unite and Connection System for Connecting Capillary Tubes, Especially for High-Performance Liquid Chromatography,” published on Mar. 15, 2012, discloses a plug unit connection system for capillary tubes, wherein a seal is provided at the interface between a capillary tube and a bushing unit, instead of at the location of a ferrule or conical fitting. However, U.S. Patent Application No. 2012/0061955 relies on the use of a pressure piece similar to a ferrule to ensure that enough axial force can be generated to obtain a seal at the tube face.
Connection assemblies which attempt to effectuate a seal for high-pressure applications can require a significant amount of torque to effectuate a fluid-tight seal, making the creation of such seals difficult without the use of additional tools and increasing the risk of damage to the fitting assembly or its components due to overtightening. Moreover, experience suggests that many users do not like to use various tools to connect or disconnect tubing from components such as those in various AI systems. It is believed that users often apply different amounts of torque to connect or disconnect tubing and the components in such systems, thus resulting in potential problems caused by over-tightening or under-tightening (e.g., leakage or loss of sealing when the fluid is under pressure).
One example of a flat-bottomed or face-sealing connection assembly is provided by U.S. Pat. No. 8,696,038, titled “Flat Bottom Fitting Assembly” and issued on Apr. 15, 2014 to Nienhuis. Nienhuis teaches a type of flat bottom assembly which includes a flat-sided ferrule, and wherein the assembly including the ferrule and the tube can be pressed against a flat bottom port. Another example of a flat-bottomed or face-sealing connection assembly is provided by published U.S. Patent Application No. 2012/0024411, titled “Biocompatible Tubing for Liquid Chromatography Systems,” which was published on Feb. 2, 2012 and was filed on behalf of Hahn et al. The Hahn et al. published patent application describes tubing having an inner layer and an outer layer, and in which the inner layer can be biocompatible material such as polyetheretherketone (PEEK) and the outer layer may be a different material, and in which an end of the tubing may be flared or otherwise adapted to have a larger outer diameter than other portions of the tubing. The current state of the art for high pressure connections in both HPLC and UHPLC is to utilize coned ports along with some form of ferrule and nut combination with tubing. The nut translates rotational torque into axial load that is translated to the ferrule. The load causes the ferrule to deform/deflect and grip the tubing, creating a seal. The tube is typically forced into the bottom of the coned port, but there is not currently a mechanism to ensure there is not a gap or space at the port bottom.
The space at the bottom of the port is a concern for those performing liquid chromatography experiments due to the potential to negatively influence the results with carry over and band broadening. Carry over is just as it sounds, analyte from one test is carried over to the next. Carry over can produce very unstable results for obvious reasons. Band broadening is when the peaks identifying a substance become less symmetric and make identification more difficult when peaks of different molecules have similar retention times.
One issue with conventional ferrules used with coned ports is that the torque required to deform/deflect is typically above finger tight levels in order to achieve UHPLC pressures (e.g., above 12,000 psi or so). It is desirable to remove tools from the lab by making them unnecessary for making and breaking fluidic connections and it is advantageous to have fittings that can be connected simply with the fingers rather than tools.
European Patent No. EP 2564104 describes a sealing system for use at high pressure. End-face seals minimize the sealing radius and therefore allow various fittings—including known ferrule fittings—to be used in high-pressure systems. End-face seals at such high pressure may require smooth surfaces, however. In order to reduce cost, an end-face preparation tool may be required to forge a dimple into the end face to mechanically deform and smooth the surface.
U.S. Pat. No. 6,056,331 describes an apparatus that is composed of three components, a body, a ferrule, and a threaded fitting. The ferrule is compressed onto a tube and a seal is formed between the tube and a device retained in the body by threading the fitting into the body which provides pressure that seals the face of the ferrule to a mating surface on the device. This seal may be used at elevated temperatures, depending on the materials used. This fitting was developed for use with micro-machined silicon wafers used in capillary gas chromatography.
U.S. Pat. Nos. 5,525,303, 5,730,943, 6,056,0331, 6,095,572, 6,056,331, 7,311,502, 8,696,038, European Patent No. EP2564104, and published U.S. Patent Application Nos. 2005/0269264, 2007/0283746, 2012/0024411, 2012/0061955, and 2013/0043677 are hereby incorporated by reference as if fully set forth herein.